1. Field of Application
The present invention relates to an apparatus for detecting knocking in the cylinders of an internal combustion engine.
2. Description of Related Art
FIG. 18 is a basic block diagram of an example of a known type of engine knocking detection apparatus, designated by numeral 10, which is made up of a bandpass filter 11, an amplifier 12, an A/D (analog-to-digital) converter 13 and a knocking judgement section 14, and receives a sensor signal SS from a knocking sensor 20, with the sensor signal SS varying in amplitude in accordance with vibration in an internal combustion engine (not shown in the drawings). The bandpass filter 11 selects components of the sensor signal SS that are within a frequency range that is known to correspond to the occurrence of knocking in a cylinder of the internal combustion engine, and the resultant filtered signal from the bandpass filter 11 is amplified by the amplifier 12 and then converted to digital signal form, before being supplied to the knocking judgement section 14. The knocking judgement section 14 evaluates that digital signal to determine whether or not engine knocking is occurring. Generally, this evaluation is based upon comparing the amplitude of the signal with a predetermined threshold value, and judging that knocking is occurring if the amplitude exceeds the threshold value.
When engine knocking is detected, appropriate processing is then applied to halt the knocking, e.g., by delaying the engine ignition timing.
With such a type of engine knock detection apparatus 10, it is necessary to ensure that noise components contained in the sensor signal SS (i.e., due to engine vibration that is unrelated to knocking) are removed before knocking judgement is performed on the signal. To achieve this, it is necessary for the passband of the bandpass filter 11 to accurately correspond to a frequency range that is characteristic of engine knocking. However in practice, the frequency of vibration caused by knocking will vary in accordance with the running condition of the engine. Hence, with the passband of the bandpass filter 11 being fixedly predetermined, it will not be optimum for extracting the knocking frequency components under some conditions of operation of the engine so that satisfactory knocking detection has not been achieved.
For that reason, it has been proposed (as described in Japanese patent publication No. 5-30664, referred to in the following as reference document 1) to prepare beforehand a plurality of respectively different filter characteristics for the bandpass filter of an engine knock detection apparatus. An appropriate one of these bandpass filter characteristics is selected in accordance with the speed of rotation of the engine.
With an alternative proposal (as described in Japanese patent publication No. 5-288114, referred to in the following as reference document 2), a plurality of respectively different filter characteristics for the bandpass filter are also prepared beforehand, for an engine knocking detection apparatus. In this case, the knocking frequency is analyzed in each of successive combustion stroke intervals, and the analysis results are used to select a bandpass filter characteristic that will be utilized in the next combustion stroke interval. However the actual variations that occur in the knocking frequency are complex, and for that reason, known types of knocking detection such as are described in reference documents 1 and 2 have not been completely successful.
The term “combustion stroke interval” is used herein to signify a unit amount of crankshaft angle increase that occurs between the starting points of two successive combustion strokes of an internal combustion engine.